Research On Microstructure Evolution And Mechanical Behavior Of 7075 Aluminum Alloy Thixoforming At High Solid Fraction

7075 aluminum alloy is widely used in aerospace field because of its excellent mechanical properties.Thixoforming take advantage of the thixotropic behavior of alloys with non-dendritic,spherical microstructures in the semisolid state,to realize the near-net shaping of complex parts,which can effectively avoid the cumbersome process of traditional processing methods and the waste of raw materials caused by following machining process.In this paper,the microstructure evolution of 7075 aluminum alloy under different parameters of reheating,such as the heating temperature,holding time and heating rate,was studied.The mechanisms of the formation of spherical grains were discussed.The mechanical behavior of 7075 aluminum alloy under high temperature and semisolid state was studied via isothermal temperature compression test.The flow behavior of the material was analyzed in combination with the microstructure characteristics during the deformation process.Corresponding constitutive models of 7075 aluminum alloy were established for high-temperature and semisolid temperature respectively.The validity of the model for the semisolid temperature range was verified by using the finite element analysis software Deform-3D to simulate the thixotropic compression.The goal of the reheating process is to obtain spherical microstructures,which is important for thixoforming.In this research,extruded 7075 aluminum alloy was used as raw material to study the effects of reheating parameters on the microstructure evolution and the formation of spherical grains.The results show that the formation of the liquid phase can stimulate the formation of spherical grains.When the heating temperature is 550?,the spherical grains are formed near the liquid phase at the grain boundary.The liquid fraction rises with increasing the heating temperature.The liquid penetrates into the grain boundaries of grains,wetting the irregular grain boundaries to form spherical grains,promoting the progress of recrystallization and the formation of spherical grains.At the semisolid temperature,the average grain size increases and the roundness increases as the holding temperature increases and the holding time increases.With a faster heating rate,the higher strain energy of the material can form more nucleation centers and refine the grains.Therefore,the extruded 7075 aluminum alloy can be directly heated to the semisolid temperature to obtain spherical microstructures and be used for thixoforming.The material flow behavior and microstructure evolution of 7075 aluminum alloy were investigated by isothermal compression experiments under high temperature and semisolid state.It was found that the compression process at high temperature includes two stages of work hardening and steady state deformation,while the compression at semisolid state includes three stages of work hardening,rheology softening and steady state deformation.Microstructural analysis shows that there is liquid segregation at the edges of the compressed samples.At higher strain rates(10 s-1),multiple peak stresses phenomenon occur because the liquid phase cannot be pooled together to form a liquid path to coordinate the deformation of the solid phase.A constitutive model was established for the peak stress at high temperature,and the calculated and experimental values of the peak stress were well fitted.An empirical constitutive model was established for the semisolid temperature.Deform-3D finite element software was used to simulate the thixotropic compression process of the 7075 aluminum alloy.The simulation results show the characteristics of the thixo-compression test.At 590 ?,simulation results and experimental results can be well fitted.In this paper,the microstructure evolution of the 7075 aluminum alloy during reheating process and the formation mechanism of the spherical grains were analyzed,which could provide help for determination and optimization of reheating parameters and further analysis of solid/liquid flow mechanisms during thixoforming process.